H'mm..
Well, my situation is a bit different, as I'm a gay woman dating a mtf gay woman.. but.. we started dating before she decided to transition, sooo.. I kind of know what the idea of dating a guy is like to a gay woman.. at least, in the identity sense.
The thing is, that when someone realizes they're straight, it's no biggie. They don't have to come out to anyone, they don't have to keep a running tally of who they are and aren't out to.. etc. All kinds of things. But when someone realizes they're gay.. it's a big part of their life, just because.. people have to be told eventually. With all the stress and complications that entails. Which may be part of the reason why we hold on to that identity more than straight people do. It's a more important part of our lives.
So when me and her started dating, way at the beginning when I thought she wouldn't transition, I realized.. my family wouldn't get it. At all. And I was really terrified that they would all think I was 'only joking' when I said I was gay, or that I'd 'changed my mind,' because that would've been an absolute lie, and part of coming out is that you don't have to live the lie anymore. And I also can't
stand when someone tells me who I am instead of letting me define myself. I didn't want to be seen as a straight woman. After coming out to basically my whole family and all of my friends, how could I explain to them I was dating a [to outward appearances] guy? Even the thought of going to a restaurant and
not hearing "what would you
ladies like?" felt strange to me.
Of course, that wasn't enough to make me choose not to date her. Not at all.

But it was definitely.. something stressing me out a little bit. I understand why your girlfriend feels worried about it.
What I thought of, and what your girlfriend might be able to do.. is I figured, that if anyone needed/deserved an explanation, I would just tell them, "Yes, I am still gay. I've just made an exception for him because he's so damn amazing." How can anyone argue with that?

And.. I wanted to mention.. I have never stopped identifying myself as a gay woman, not since I came out to myself 6 years ago. Never stopped being gay. It's not disrespect.. it's who I am.